U.S. Pat. No. 4,188,826 and DE 27 23 999 C2 disclose a device for electric measurement of the level of electrically conductive fluids. An elongated electrode is immersed into the liquid there. The resistance of the electrode itself should be low with respect to the transition resistance of the electrode to the liquid. A voltage is applied to two ends of the electrode. Due to the differing heights of the potentials along the electrode in contact with the liquid, stray currents develop in the liquid. The potential of the liquid outside of the area of the stray currents corresponds to the potential of the center of the part of the electrode immersed in the liquid, with the stray currents running exactly symmetrically with the center of the immersed part of the electrode. To detect the potential outside of the area of the stray currents of the immersed electrode, the container wall, which is at a great distance, is provided. With this measurement arrangement, this yields a linear path between the measured voltage and the filling level.
EP 0 658 511 B1, DE 43 22 088 A1, AT 338640 and EP 0 598 892 B1 disclose devices for detecting the liquid levels in bottles in which electrodes can come in contact with the liquid so that the filling level can be deduced by the short circuit between the two contacts. A linear filling level measurement is difficult or impossible here, i.e., it is possible only with a great many individual resistors and corresponding contacts. However, these contact measurements have the advantage that a conductive container wall is not needed.